Patent GB-798413-A (GB)
Title: Improvements relating to electrical apparatus for indicating the departure of a variable quantity from a given range of values
Inventor: ALEXANDER WILLIAM HECTOR
IPC: G01P1/10
Filed: 1954-10-28
Abstract: 798,413. Voltage measurement. FERRANTI, Ltd. Oct. 21, 1953 [Oct. 28,1954], No. 31083/54. Class 37. [Also in Group XXXVIII] Electrical apparatus indicating any departure of a quantity from a given range of values containing a datum value comprises means for representing the datum and quantity values by respective voltages, apparatus for deriving a voltage representing in sense and magnitude the difference between the datum and quantity values, first and second polarized bias relays whose dual operating windings are respectively connected for energization by the difference voltage in such manner that the latter sets up fluxes opposing the bias flux of one relay and reinforcing the bias flux of the other as determined by the sense of the difference; the bias flux values being dependent on the range of values of the input quantity in such manner that said other relay operates when the quantity departs from such range and the relays being connected to energize circuits displaying indications of such departure in either respective sense. In Fig. 1, a shaft 13 driven by an airspeed indicator adjusts potentiometer 8 to derive an airspeed voltage which is subtractively combined with a preset datum airspeed voltage from potentiometer 7 in series with first windings 1, 2 of relays A/1 and B/1 whose second windings 3, 4 are series energized from range setting potentiometer 15, excited in series with variable resistance 16 shunted by normally closed contact D1. The series of connection of the relay windings are such that the fluxes set up in the relay cores by the respective series windings are such that those due to one pair add and those due to the other subtract so that neither relay is operative while the difference between the input and the datum lies within the allowable range while relay A/1 or B/l operates when the input quantity voltage is either greater or less than the datum voltage, respectively. In Fig. 2, capacitance shunted windings 32, 33 of relay C/1 are connected alternatively in series with resistances 34, 36 across an energization source by changeover contacts C1 of the relay, which automatically changes over continuously and sequentially at a frequency set by resistances 34, 36 and capacitances 35, 37 respectively. Relay D/3 across resistance 34 alternately operates and releases at the changeover frequency, and its normally closed contact D1  accordingly pulsating by open circuits variable resistance 16 in series with potentiometer 15 whereby the range value is expanded symmetrically on each operation of D/3 to a degree set by resistance 16. When the airspeed is within the datum range, relays A/1 and B/1 are inoperative, so that green lamp 27 is energized over the released contacts A1, Bl, variable resistance 24, and resistance 30 alternatingly shunted by contact D2 to flash the lamp continuously bright or dim. An increase of airspeed outside the datum range set by potentiometer 14 but within the expanded range set by resistance 16 initiates a difference voltage of such polarity as to operate relay A/1 while relay B/1 remains inoperative during the intervals wherein contact D1 is closed, while during intervals wherein D1 is open both relays are inoperative. Thus contacts A1 alternate continuously, while contacts B1 remain normal, so that green lamp 27 flashes alternately with amber lamp 21 ; contacts D2 alternating synchronously with contacts A1 to short circuit resistances 23, 30 in sequence so that each lamp flashes brightly in turn. Similarly, a decrease in airspeed outside the datum range of potentiometer 15 but within the expanded range set by resistance 16 alternately operates and releases relay B/1 to flash green lamp 27 alternately with red lamp 26. When the airspeed increases or decreases outside the expanded range set by resistance 16, relay A/1 or B/1 is continuously operated to energize lamp 27 or 21 and to prepare a circuit over rectifiers 40 and 41 to relay E/4.. Closure of the contact D3 energizes relay E/4 whih locks over contact E3 so long as the air-speed value remains outside the expanded range. Contacts El, E2 close, and increase the frequency of alternation of relay C/1 by short circuiting a section of resistances 34, 36, while contacts E4 close to short circuit resistance 22 and contacts D2 alternate as before so that the relevant lamp flashes brighter and dimmer than normal at a higher frequency than before. The normal lamp brightness is controlled by variable resistance 24 while the operating ranges may be asymmetrical with regard to the datum, e.g. by imposing a standing magnetic flux bias on either relay by a permanent magnet. The input quantity may be altitude, and the datum quantity may be continuously varied, e.g. in accordance with the varying altitudes of a glide path to be followed in landing an aircraft. Audible tone generators may replace coloured lamps and the apparatus may respond to any quantity which may be represented by voltage.